1. Field of the Invention:
The present invention relates to a pattern recognition method and, particularly to an improved pattern recognition method which precisely recognizes confusing characters, and phoneme that constitutes voices and corresponding to each symbol constituting a language.
2. Description of the Prior Art:
According to a conventional pattern recognition method such as a method for recognizing letters and voices, an input pattern and a standard pattern are subjected to comparison, and a pattern having a category name of the standard pattern having an optimum degree of identification is introduced.
In recognizing the letters, when, for example, a Chinese character " " (large) is introduced, the comparison can be generally performed well with respect to the following Chinese characters " " (dog) or " " (thick), in addition to a standard pattern Chinese character " " (large). In recognizing voices, when, for example, the sound /t/ is introduced, the comparison can be usually performed well with respect to the same voiceless stop consonants such as /p/ or /k/ or with respect to /d/, /z/, or /s/ having the same place of articulation. Therefore, there is a great probability for developing erroneous recognition among such similar patterns, and the ability to perform accurate recognition is decreased.
In recognizing phonemes, for example, in voice produced by a physical phenomenon such as vibration of the vocal organs, the phonemes which constitute the voice produced under limited physical conditions such as length of the vocal organs, may appear to be greatly affected by the preceding or succeeding phoneme and the speed of speech.
Therefore, it is very difficult to precisely recognize the phoneme.
In order to overcome the above difficulty, a method was proposed, according to which a spoken word containing deformed phonemes was compared as a practical recognition unit with a standard pattern.
According to the above method, however, it was necessary to prepare standard patterns of such large units as spoken words consisting of a combination of phonemes and, hence, it was necessary to store in the memory the standard patterns related to spoken words that were to be recognized. Since the memory of a tremendous capacity was necessary, it was virtually impossible to construct a voice recognizing apparatus which is capable of recognizing any voices like a so-called voice typewriter.
In order to recognize any voices, therefore, it becomes an essential requirement to perform the recognition on the phoneme level.
As mentioned above, however, the recognition on the phoneme level presents the following problems:
(1) It becomes difficult to perform the recognition as the phoneme is deformed.
(2) A phoneme has a length considerably shorter than that of a word, which causes confusion among different phonemes.
(3) Voice is continuously produced with the passage of time, and it is necessary to cut out the phoneme as a sectional pattern from the continuous voice pattern. It is, however, very difficult to properly cut out the sectional patterns.
With respect to the above-referenced third problem a system called the continuous DP (dynamic programming) matching method has been proposed in order to continuously perform the matching of the introduced voice pattern with the standard pattern without the need of cutting the continuously produced voice pattern after a predetermined period of time, and the effectiveness of the continuous DP matching method has been confirmed. See Continuous Speech Recognition by Continuous DP Matching" by Ryuichi Oka, Technical Report of Acoustic Society of Japan, S78-20.
To cope with the above-referenced first and second problems, on the other hand, methods have been proposed in order to:
(i) Increase the kinds of characteristic parameters so that slightest differences among the phonemes can be detected;
(ii) Prepare standard patterns to emphasize consonant portions of the phonemes: and
(iii) Improve the matching method so that it is less affected by the deformed phonemes.
None of the above methods, however, have produced satisfactory results.